Electrolytic refining of aluminum



April 21, 1925.

w. HOOPES ET AL ELECTROLYTIC REFINING OF ALUMINUM Dec. 21, 19.22

anveutow In" #001 55, EC. 1 7%? v and JD Fawn/#0 5 351 their 6141mm MG,a f-f M v Patented Apr. 21, 1925.

UNITED STATES PAITE-KNIT OFFICE.

WILLIAM HOOPES, OF PITTSBURGH, AND FRANCIS FRARY JUNIUS D. v1 1D- WARDS,OF OAKMONT, PENNSYLVANIA, ASSIGNORSTO ALUMINUM COMPANY OF AMERICA, 01PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA.

ELECTROLYTIC REFINING OF ALUMINUM.

Application filed December 21, 1922, Serial No. 608,285.

To all whom, it may concern:

Be it known that we, WILLIAM Hoorns, FnANois C. FRARY, and JUNrUs D. En-WAnos, all citizens of the United States of 5 America, the said WILLIAMHoorns residing at Pittsburgh, and the said FRAiNoIsC. FRARY and J UNIUSD. EDWARDS residing at Oakmont, all' in the county of Allegheny andState 'of Pennsylvania, have. invented certain new and usefulImprovements in Electrolytic Refining of Aluminum, of which thefollowing is a full, clear, and. exact description. 1 t

' This invention relates generally to the refining of aluminum byelectrolytic removal or separation of themetalrfrom an alloy or amixture thereof with other substances, and relates more particularly toa refining process in which the electrolyte floats, in a molten state,upon-the molten alloy, the latter being used as anode and the aluminumremoved being deposited on a layer of molten aluminum as cathodefloating on the electrolyte."

In an extensively used aluminum reduction process, wherein aluminumisproduced by reduction of its oxid, a cryolite bath or electrolyte .isused, but in the herein described process, in which it is desired tohave the refined metal float on the electrolyte, such a. bath, thoughotherwise operable, cannot be used alone for the purpose, since in themolten state it is lighter than aluminum and hence would allow thelatter to sink. While other salts may be'added to give adequate density,such addition easily leads to difficulties of various sorts, resultingin failure to produce pure metal, and no prior inventor, so far as weare aware, has attained commercial success in this way or in any other.\Ve have found that success may be realized and a bath producedcombining the desired characteristics of density, fluidity, stability,capacity for dissolving alumina, conductivity, and selective solutionand deposition of aluminum during electrolysis, by adding to cryolite(or preferably to a mixture of aluminum and sodium fluorids richerinaluminum fluorid'than is cryolite) salts of the alkali earth metals.These metals are less easily precipitated from the bath than aluminumand we find certain of their salts to be the only generally availablematerial suitable for the purpose of increasing the density of the bath.We find that the only materials giving in high degree the othernecessary or desirable characteristics are the fiuorids of these metals(barium, strontium, calcium and magnesium). These form with cryolite,mixtures which are readily fusible, but our experience indicates thatmagnesium fiuorid has less effect than any of the others in increasingthedensity of the bath. Barium fluorid has been suggested as aningredient of electrolytes for other purposes, but so far as weare awareno one has heretofore suggested their use for refining aluminum orhasgiven, with respect to the properties of barium fluorid and cryolitemixtures, the information needed for their successful'use. We-havefound, however, that within certain limits, mixturesof the substancesmentioned furnish excellent electrolytes, and that such a bath does notlose aluminum fluorid by volatilization at the working temperature to aserious extent. Its electrical conductivity and power of dissolvingalumina are also goo Moreover, the use, in an electrolyte, of a halogensalt other than a salt of fluorin, is in general disadvantageous and insome cases impractical. This is especially true in processes designed toproduce substantially pure aluminum, for the reason that chlorids inthebath cause solution and deposition of other substances than aluminum, asfor example zinc, iron, titanium, copper, and silicon, with the resultthat if the anode contains any of these substances the cathode metal maybe contaminated therewith to an intolerable degree. Stated otherwise, ifchlorin anions are present in the bath, it is difficult and in somecases impossible to prevent the solution of other substances thanaluminum from the anode and their deposition on the cathode. On theother hand the presence of oxygen anions is permissible. Accordingly anadvantageous feature of the present process is the use of a bath orelectrolyte which may contain fluorin or oxygen anions or both but issubstantially devoid of chlorids, and which therefore, as our experienceshows, exerts a truly selective action upon the anode alloy indissolving aluminum therefrom substantially to the exclusion of othermetals under conditions where it would be expected that considerableamounts of other metals would be dissolved, and where, if substantialamounts of chlorids are present, they do actually dissolve.

' \Ve have determined the density of aluminum of 99.7 5 per cent purityto be approximately 2.29 grams per cc. at 1000 C. At this temperature,the density of molten cryolite is approximately 2.10 grams per cc. Inorder to increase the density of the cryolite, so that this aluminumwill float on it at the temperature mentioned, we have found that theremust be added about 20 parts of barium fluorid to each 80 parts ofcryolite. If calcium fluorid were to be used, about 40 parts would berequired to 60 parts of cryolite. While the above mentioned mixture ofcryolite and barium fluorid is completely molten at temperatures above965 C., mixtures of calcium fluorid and cryolite containing about 40 percent or more of the former by weight require a temperature above 1000 C.to keep them molten. This would make the working temperature of the bathundesirably and in some cases impracticably high. On the other hand,mixtures of cryolite and one or more fluorids of alkali earth metalshaving atomic weights above 80 are in general suitable; though ofcourseradium is to be excepted, for obvious reasons. Thus mixtures ofcryolite and barium fluorid containing between about 20 and 60 per centof the latter constituent by weight are readily fusible at temperaturesbelow 1000 C., and have densities in the molten state ranging betweenabout 2.38 and 3.15 grams per cc. at 1000 C. Even the heaviest of thesemixtures is sufficiently light to float on any of a number of .moltenaluminum alloys suitablefor use as anode in the electrolytic refining ofaluminum.

A fused mixture of cryolite and strontium fluorid containing betweenabout 20 per cent and 60 per cent of the latter also lies within theproper range of densities for our purpose, but such mixtures containingmore than about 40 per cent of strontium fluorid are in general not soreadily fusible as mixtures containing corresponding amounts of bariumfluorid.

In operating with a bath of the type described above, it has beenobserved that there is a considerable deposition of metallic sodium atthe cathode, and that this sodium, being in vapor form, passes upthrough the molten aluminum cathode and causes difliculties inconnection with the leading out of the current from the cathode metal.Carbonaceous rods used to carry the current out of the floating layer ofaluminum are attacked and eventually disintegrated by the sodium, but ithas been found that this difficulty may be decreased, withoutinterfering seriously with the density of the molten bath, by increasingthe proportion of aluminum fluorid as compared to sodium fluorid,although such an increasedoes perceptibly increase the electricalresistance of the bath.

As an example of baths or electrolytes which have been found by actualuse to be suitable for our purpose, We recommend one havingapproximately the following composition:

Barium fluorids. 30 to 38 per cent Sodium fluorid 25 to 30 per centAluminum fluorid 30 to 38 per cent Alumina 0.5 to 7 per cent Calcium andmagnesium fluorids (present as unavoidable impurities)--- about2per centSuch a bath is completely molten at all temperatures above about 900 C.and enables the refining to be carried on at about 950 C. At thistemperature the bath is satisfactorily stable, has good electricalconductivity and satisfactory density, and is capable of dissolving asatisfactory amount of alumina.

It will be understood that'the sodium and aluminum fluorids of theelectrolyte may be supplied, in part at least, by cryolite, which has acomposition generally accepted as 3NaF.AlF

It has been observed that the density of these molten baths decreasesmore rapidly with rising temperature than does the density of the moltenaluminum, and hence it is advantageous to provide a distinct margin ofsafety between the two densities, so that if the cell should becomeoverheated the bath will notbecome so light as to permit the aluminumtop layer to sink to the bottom. The density of bath of the abovementioned composition lies between about 2.5 and 2.7 at 950 C., andbetween about 2.4 and 2.6 at 1100 (1., and accordingly pure aluminumwill float on the bath at these temperatures, since the density ofaluminum at 950 C. is about 2.30 and at 1100 C. is about 2.26 grams percc.

A bath containing cryolite and 00 per cent of barium fluorid (instead ofthe chlorid) will dissolve between 4 and 5 per cent of alumina; and asimilar bath containing 40 per cent barium fluorid, which would have aspecific gravity of about 2.7 3, will dissolve between 8 and 9 per centof alumina at 1000 C. Electrolytes of such type are thereforeadvantageous for the following reasons.

Alumina becomes more soluble in any of the cryolite baths as theirtemperature 'is raised, but if alumina be added until the bath issaturated it will be found that a small drop in temperature will causesome of the alumina to precipitate out as coron-- dum or incorundum-like form, with which more or less bath will be mechanicallyassociated. In an operating cell the portion of the bath adjacent to thewalls, together with. that portion forming the top crust or coming incontact with it, is usually at a temperature distinctly lower than thatof the main body of the bath, so that if suflicient alumina enters tosaturate this main body the natural circulation will cause a depositionof part of the alumina on the walls of the cell in theform of athickened crust. Practical operation of such a bath has shown that whenthe alumina has once crystallized in this form, it is extremelydiflicult to re-dissolve it in the bath. A certain amount of such cruston the interiorof the cell is desirable for its thermal and electricalinsulating properties (as described more fully in the copendingapplication of \Villiam ll'oopes, Junius 1). Edwards and Basil T.Horstield, Serial No. 608,289, filed December 21, 1922) 'but to preventthe formation of an undesired amount of this deposit, which wouldotherwise gradually fill up the cell and interfere with the operation,it is important to keep the alumina content be- 1 low the saturationpoint. In order to maintain this condition it is therefore desirable, inpractice, to have the bath capable of dis solving considerable alumina,so as to allow for unavoidable variations incident to operatingconditions; for the reason that any of the following causes may operateto add alumina to the bath: (A) The hydrolysis of aluminum fiuorid bymoisture. Capillary action continually brings some of the bath upbetween the top metal layer and the side of the cell, so that it formsa; crust on the top of the metal, where it is maintained at a hightemperature and exposed to the air. The excess of bath above that whichcan solidify to form this crust drips back through theinetal from timeto time, and when the crust is disturbed or broken, parts of it sinkthrough the metal, and return to the main body -of the bath. (B) Thereaction of sodium oxid (or hydroxid) with aluminum fluorid. More orless sodium is always liberated at the cathode, and. some of it risesthrough the metal .layer', probably in the form of vapor, and reachesthe top crust where it is oxidized by contact with the air. (C) Directoxidation of the floating aluminum layer by air penetrating throughcracks in the top crust. (D) Alumina dust, which is always present in aplant in which the Hall process of producing aluminum is operated, willsettle on the crust of the refining cell if it is operated in the sameplant.

So important is it to have the bath unsaturated that in actualcommercial operation it is generally necessary to remove alumina fromtime to time. This may be conveniently done in various ways, as by oneor another of the methods described and claimed in a copendingapplication of \Villiam Hoopes and FrancisC. Frary.

, The cell preferred for use in our aluminum refining process is of thetype described and claimed in the copending application of \VilliamHoopes, Serial No. 608,287, filed December 21, 1922- One form of thiscell is illustrated in the drawings annexed hereto, in which Fig. 1 is aplan View of the cell.

Figs. 2 and 3 are cross sections on lines 22 and 8-3,-respectively, ofFig. 1.

' Figs. 4; and 5 are'detail cross sections on lines t4i and 5- 5,respectively, of Fig. 1,

illustrating the water connections to and from and between the'waterjackets.

Fig. 6' is a detail cross section on the line 6-6 of Fig? 1, showing themethod of con-. nectingthe upper electrodes to the negative busbars.

Fig. 7 is adetail cross section on the same plane as Fig. 2,illustrating the method of securing the upper and lower shell sectionstogether to give adequate mechanical strength without connecting the twoelectrically. I

The lower shell or shell section 10 is preferably made of steel in theform of a cylindrical vessel of considerably greater diameter thanheight, and at or near its top it is provided with a water jacket 11which is most conveniently formed by providing at the upper edge of theshell section an outwardly extending flange 12 of suitable width, and aflaring or conical ring 12 welded or otherwise hermetically joined tothe underside of the flange and to the body of the shell below.

Above the lower shell section 10 is an I upper shell section 13 whichmay also be of 7 steel and formed with hollow walls to provide an upperwater jacketl l. The inner surface of the upper shell section ispreferably flaring, as indicated. To keep the sections electricallyinsulated or separated from each other a fiat ring or gasket 15, of asbeused. If the water jackets are used, asin most cases they will be, thebushings and washers will not be subjected to a high temperature andhence they can be made of practically any insulating material which willnot soften at temperatures below 100 C. and which can withstand thecrushing stress exerted by the studs. Mica has been found to answer thepurpose very satisfactorily.

Suitable water connections for the water jackets are provided, and forthe sake of simplicity and convenience these connections may be soconstructed and arranged that the water flows through the'two jackets 1nsuccession, preferably through the lower jacket first. For this purposethe jacket 11 may be provided at the bottom with an inlet nipple 20connected by a pipe 21 to any convenient source of water, not shown, andat the top (to prevent pocketing of air) with an outlet nipple 22connected by a pipe 23 to the inlet nipple 24 by which water from thelower jacket is led into the bottom of the upper. The latter is equippedwith an outlet nipple 25 (at the top to prevent air pocketing) which maybe connected to a waste pipe 26 by means of a pipe 27. To avoidelectrical grounding the pipes 21 and 27 may consist of rubber hose, asmay also the pipe 23 to keep the two shell sections electricallyseparate. The water used when the jackets are connected should be ofsuflicient purity to prevent material flow of current from one shellsection to the other at the voltage employed in operation.

In the bottom of the lower shell section a layer 28 of heat-insulatingmaterial may be provided, as powdered bauxite, alumina, magnesia, orrefractory bricks, to decrease or minimize loss of heat through thebottom of the cell, and above this layer is a bottom lining 29 ofrefractory electrically conducting materiaLpreferably carbon, andpreferably having a cavity or depression in its upper portion to receivethe alloy or other material to be refined. The bottom lining can beconveniently and satisfactorily made by tamping into the shell a mixtureof tar, pitch and granular or'powdered coke, at a temperature highenough to make the mass plastic, and placing the shell and contents inan oven in which the temperature is gradually raised, say to about 6000., for the purpose of baking and solidifying the can bonaceous mass.

Good electrical conne:tion may be provided between the shell and itsbottom lining by means of metal collector plates 31, welded to the,inner surface of the shell so as to be electrically and mechanicallycontinuous therewith. These plates extend inwardly into the bottomlining, which is molded around them. At the'plane of the collectorplates the shell may be provided on the outside with metal contact pads32, preferably welded to.the shell so as to be mechanically andelectrically continuous therewith, to which pads busses or busbars,

fining operation these busses are connected to the positive terminal orpole of the source, so that the current enters the cell at the bottom.The carbon bottom or bottom-lining, 29, constitutes What may forconvenience be termed the lower electrode of the cell.

The upper electrode may be multiple, as indicated, composed preferablyof a suitable number of short thick rods 34 of graphite, arrangedvertically and having copper or other metal. rods 35 threaded orotherwise suitably secured to the tops of the electrodes. These metalrods serve to support the upper electrodes and convey current to andfrom the same, and for this purpose they may be releasably andadjustably secured, as by means of clamps 36, to metal busbars 37extending horizontally across the cell.- For convenience of access to'the graphite cylinders, for adjustment, replacement, etc., the busbarsmay be arranged at two or more different levels, as indicated, and maybe supported on and secured to a plurality of legs 38 to form a rigidframe work. The latter may rest on the upper shell section, in whichcase they are insulated from the shell section, as by any convenient andsuitable means, not shown.

It is recognized that, strictly speaking, the aluminum layer floating onthe bath and the layer of alloy underlying the bath, are the upper andlower electrodes, respectively, but these layers are termed herein thecathode and the anode, and hence it is deemed permissible as well asconvenient to refer to the graphic cylinders and the carbon bottom-.lining, or their equivalents, as the upper and lower electrodes.

Metal or other molten material ma be withdrawn from the upper portion 0the cell through a tapping notch 39, which may be closed by means of anysuitable refractory material. Molten'metal or other material may bewithdrawn from the lower port of the cell through a port or tapping hole40, normally closed by means of a plug of dense charcoal or othersuitable material.

On the inside of the cell is a side-lining 45 extending upwardly fromthe carbon bottom 29, over the joint between-the shell sections and wellup toward or even over the top of the upper shell section. Inpractically all cases this side-lining should be thermally andelectrically insulating, to decrease or minimize the conduction of heatto the water jackets as well as to prevent bypassing of current aroundany art of the cell contents undergoing electro ytic treatment in therefining operation. The linin should be refractory'enough to remain soliat the temperatures to which it is exposed in the electrolytic refiningoperation. Under these conditions a lining composed of or formed from amixture of metal fluorids and alumina, as more fully explained in thecopending application of \Villiam Hoopes, J unius D. Edwards, and BasilT. Horsfield, Serial No. 608,289, filed December 21, 1922, has beenfound highly satisfactory in practice.

In the refining process the aluminum alloy or mixture of aluminum andother substances lies in molten form in the bottom of the cell asindicated at 46. Floating on this is a layer 47 of fused bath orelectrolyte, and on the latter is a layer 48 of molten aluminum, withthe upper electrodes extending into it far enough to insure goodelectrical contact. The molten layers are preferably established in thecell by successively pouring the previously fused materials into place,using for the original aluminum layer the purest metal convenientlyavailable. The cell may also be putin operation in the following manner.I

The upper electrodes are lowered into contact with the carbon bottom andcurrent is sent through them to the latter, thereby generating heat andfusing a small quantity of powdered o'r granulated bath material placedaroundthem. The upper electrodes are raised as the melting proceeds, andadditional bath material is supplied, until a sufficient body of fusedelectrolyte has been produced. The molten anode alloy or mixture is thenpoured in. Almost any aluminum alloy can be used which is denser thanthe molten bath and which will remain mobile during the refining.operation. Preferably. however, we use an alloy of which the principalcomponents are aluminum and copper. The alloy should be suppliedinsufficient amount so that "it will remain in an electricallycontinuous layer'on the bottom of the ell throughout the refiningoperation. A bath layer of sufficient depth should be used so that thetop metal (the pure aluminun1)'will in no case come into contact withany portion of the side crust which has pre- Yiously been covered by theanode alloy. It is to be noted in this connection that the changes incomposition of the anode alloy, incident to the refining operation,cause corresponding changes in its volume and in the position ofthe'upper and lower surfaces of the bath layer. Molten aluminum,preferably the purest obtainable, is placed on the molten bath, to serveas cathode.

The refining process can now be begun, with the alloy as anode and thetop metal as cathode, the current being led from the before thewithdrawal.

top metal by means of gra hite electrodes dipping into it. Under t eseconditions aluminum is dissolved out of the anode alloy and deposited inmolten form on the cathode. This is continued until the desired amountof aluminum has been re moved from the anode and. added to the cathode.A portion of" the top metal is then removed and the impoverished anodealloy is withdrawn through the tap hole 40, fresh anode alloy in themolten state being supplied in any convenient way, preferably such thatthe refined metal floating on'the bath will not be contaminated. Thisopera; tion .may be conveniently performed by means of acarbon funnel,which after being preheated, is let down .until it early reaches thebottom of the cell, whidlhhhas preferably been cut out of the circuit.The refined metal entrapped in the funnel ay be dipped out with a handladle, after which the fresh anode alloy is poured in. The funnel isthen lifted out and the refining process resumed. The fresh anodealloyintroduced is preferably suflicient in amount to raise the bath and topmetal until the surface of the latter is at the same 1 el as Theseoperations may be repeated from time to time as necessary orv desirablewithout seriously interrupting the refining process, which otherwise cango on continuously.

The energy-efficiency in electrolytic. processes of refining aluminum isdependent largely upon the perfection of the measures taken forpreventing escape of heat. Theoretically almost no energy is requiredfor the refining; but practically, in the absence of some other adequatesource of heat, sufficient electrical energy must be expended tomaintain the anode, the bath, and the cathode in a fused condition, andconsequently the amount of electrical energy whichmust be supplied isalmost exactly the equivalent of the heat permitted to escape. After theheat insulation of the cell has been perfected to the maximumpracticable extent, nothing furtheFan be accomplished in limitation ofthe amount of heat escaping from a heated body of given dimensions, andwith the minimum heatloss the energy input required by the cell willalso be a minimum. In the interests of power economy the cell should beoperated at the lowest practicable voltage. Accordingly the electrolyte,which furnishes the major portion of the resistance, should be in asthin a layer asis permissible, and it has been found that a layer from 2to 1 inches thick is in general satisfactory. Vith a bath or electrolyteof any predetermined workable depth, the current density permissiblevaries between a lower limit which is sutlicient to maintain the anode,the bath and the cathode in a molten state.

- per square foot.

and an upper limit at which volatilization of the bath is excessive orat which too large a proportion of anode impurities go into solution.These limits, with the various bath-compositions which we have foundsatisfactory, are approximately 800 C. and 1100 0., respectively, with apreferable working temperature of about 950 C. Other miscible lowerlimit of current density also depends to a considerable extent upon thesize of the cell, since the ratio of minimum of about 780 amperes and aper-' missible maximum of about 1250 amperes,

With the preferred current density mentioned, the total voltage betweenthe terminals of the cell may be about 6 volts. Larger cells may beoperated with lower current densities and at lower voltages, and byvarying the size of the cell, the composition of the bath, theconductivity of the bath, and the effectiveness of the heat insulation,the present electrolytic refining process is workable commercially withcurrent densitiesbetween about 500 and about 2500 amperes per squarefoot ofcross section of the bath. In general the lower practicable limitof voltage is about 3.5 volts and the upper limit is of courseindefinite.

The current is led from the molten aluminum cathode preferably by meansof the electrodes or current-carrying members described in the copendingapplication of Francis C. Frary, Serial No. 672,867, filed November 5,1923. These are made of graphite, in the form of short, thick rods orcylinders, and may be protected against oxidation in the air by means ofa non-oxidizable coating which may consist of molten bath materialapplied to the rods as a thin layer and allowed to freeze thereon.

It is to be understood that the invention is not limited to the detailshereinspecifically illustrated or described but can be carried out inother. ways and with other appgratus, without departure from its spirit.

the p nded claims the expression fluori of an alkali earth metal havingan atomic weight greater than 80, is used in a generic sense to includeeither barium or strontium fluorid or both, each of these metals havingan atomic weight greater than that named. Similarly, in the claimsspeciing barium fluorid it is to be understood at for a part of suchfluorid, strontium fluorid may be substituted in suitable proportion.

We claim- A 1. In the electrolytic refining of aluminum, the stepscomprising establishing between a lower layer of molten metal containingaluminum, as anode, and an upper layer of molten aluminum as cathode, anintermediate layer of molten electrolyte of greater density than themolten aluminum and composed essentially of fluorids and substantiallyfree from chlorid, and passin current from the anode metal through theelectrolyte to the aluminum cathode whereby aluminum is removed from theanode metal and deposited in the molten state on the cathode. I

2. In the electrolytic refining of aluminum, the steps comprisingestablishing between a lower layer of molten metal containing aluminum,as anode, and an upper layer of molten aluminum as cathode, anintermediate layer of molten electrolyte of greater density than themolten aluminum and composed essentially of fluorids and substantiallyfree from other compounds except oxid, and passing current from theanode metal through the electrolyte to the aluminum cathode wherebyaluminum is removed from the anode metal and deposited in the moltenstate on the cathode.

3. In the electrolytic refining of aluminum, the steps comprisingestablishing between a lower layer of molten metal containing-aluminum,as anode, and an upper layer of molten aluminum gas cathode, anintermediate layer of molten electrolyte containing aluminum and sodiumfluorids aid between 20 and 60 per cent, approximately, of fluorid of analkali earth metal having an atomic weight greater than and passingcurrent from the anode metal through the electrolyte to the aluminumcathode whereby aluminum is. removed from the anode metal and depositedin the molten state on'the cathode.

4. In the electrolytic refining of aluminum, the steps comprisingestablishing between a lower layer of molten metal containing aluminum,as anode, and an upper layer of molten aluminum as cathode, anintermediate layer of molten electrolyte substantially free from chloridand containing aluminum andsodium fluorids andbetween 20 and 60 percent, approximately, of fluorid of an alkali earth metal having anatomic weight greater than 80; and passing current from the anode metalthrough the electroremoved from the anode removed from the anode ingaluminum as anode, and an upper layer of 'molteiLal'uminum as cathode,an intermediate layer of moltenelectrolyte containing aluminum andsodium fluorids and between 20 and 60 per cent, approximately, ofbarium. fiuorid; and passing current from the anode metal throughthe'electrolyte to the aluminum cathode whereby aluminum is metal and deposited in the molten state on the cathode.

6. In the electrolytic refining of aluminum, the steps comprisingestablishing between a-lower layer'of molten metal containing aluminum,as anode, and an upper layer 7 f-1nolten aluminum as cathode, anintermediate layer of molten electrolyte substantially free from chloridand containing aluminum and sodium fluorids and between and 60 per-'eent, approximately, of barium fiuorid; and passingcurrcnt from theanode metal through the electrolyte to the aluminum cathode wherebyaluminum is metal and deposited in the molten state on the cathode.

7. In the electrolytic refining of aluminum, the steps comprisingestablishing between 21 lower layer of molten metal containing aluminumas anode, and an upper layer of molten aluminum ascathode, anintermediate layer of molten electrolyte containing between 20 and percent, approxi mately, of barium fluorid, -and containing sodium andaluminum fluorids in such proportions that the ratio of aluminum fiuoridto sodium liuorid is greater than cryolite; and passing current from theanode metal through the electrolyte to the aluminum cathode wherebyaluminum is removed from the 'anode metal and deposited in the moltenstate on the cathode.

i 8.. In the electrolytic refining of alumi- Y num, the steps comprisingestablishing between a lower layer of molten metal containing alun'nnum,as anode, and an upper layer of molten aluminum as cathode, anlntermediate layer of molten electrolyte containing cryolite, additionalaluminum fluorid, and barium fluorid, the latter fiuorid constitutingbetween 20 and per cent, approximately. of the whole; and passingcurrent from the anode metal through the electrolyte to the aluminumcathode whereby aluminum is removed from the anode metal and depositedin the molten state on the cathode.

9. In the electrolytic refining of aluminum, ie steps comprisingestablishing between a lower layer of molten metal containing aluminum,as anode, and an upper layer of molten aluminum as cathode, anintermediate layer of molten electrolyte substantially free from chloridbut containing between 20 and 60 per cent, approximately, of bariumfluorid, and containing sodium and aluminum fluorids in such proportionsthat the ratio of aluminum fluorid to sodium anode metal and depositedin the molten state on the cathode.

. 10. In the electrolytic refining of aluminum, the steps comprisingestablishing between a lower layer of molten 'metal containing aluminum,as anode, and an upper layer of molten aluminum as cathode, anintermediate layer of molten electrolyte substantially devoid of chloridand containing cryolite, additional aluminumfiuorid, and barium fluorid,the latter fluorid constituting between 20 and 60 per cent,approxin'iately, of the whole; and passing current from the anode metalthrough the electrolyte to the aluminum-cathode whereby aluminum isremoved from the anode metal and deposited in the-molten state on thecathode.

11. In the electrolytic refining of aluminum, the steps comprisingestablishing a lower layer of molten metal containing aluminum, asanode, and having a materially higher density than aluminum, an upperlayer of molten substantially pure aluminum as cathode, and anintermediate la er of fused electrolyte composed of a suitab e mixturemolten at 1050 C., capable of dissolving a substantial amount of aluminaand having a density less than that of the anode metal but greater thanthat of the aluminum cathode metal; and passing current between theanode and the cathode.

12. In the electrolytic refining of aluminum, the steps comprisingestablishing a lower layer of molten metal containing aluminum, asanode. and having a materially higher density than aluminum, an upperlayer of molten substantially pure aluminum as cathode, and anintermediate layer of fused electrolyte composed of a suitable mixturemolten at about 950 C., capable of dissolving a substantial amount ofalumina, and having. a density less than that of the anode metal butgreater than that of the aluminum cathode metal; and passing currentbetween the anode and the cathode;-

13. In the electrolytic refining of'aluminum, the step comprisingelectrolyzing an impure aluminum-bearing molten metal or alloy ofrelatively high density as anode, with a superimposed molten bathcontaining cryolite and suliicient barium fluorid to We the bath asubstantially higher density t an aluminum and still permit the bath todissolve a substantial amount of alumina.

14. In the electrolytic refining of aluminum, the step comprisingelec'trolyzmg an impure aluminum-bearing molten metal or alloy ofrelatively high density, as anode, with a superimposed molten batl1substantially free from chlorid and containing cryolite and sufiicientbarium fiuorid to give the bath a substantially higher density thanaluminum and still permit the bath to dissolve a substantial amount ofalumina.

15. In the electrolytic refining of aluminum, the step comprisingelectrolyzing an impure molten aluminum-bearing metal or alloy ofrelatively high density as anode, with a superimposed molten bathcontaining aluminum, sodium, and barium fiuorids, in proportions adaptedto give the bath a greater density than that of aluminum and permitsolution of a substantial amount of alumina.

16. In the electrolytic refining of aluminum, the step comprisingelectrolyzing an impure molten aluminum-bearing metal or alloy ofrelatively high density as anode, with a superimposed moltenbathcontaining aluminum, sodium, and barium fiuorids, in pro ortions adaptedto give thebath a greater ensi than that of aluminum and permit solutionof a substantial amount of alumina, the ratio of aluminum fluorid tosodium fluorid in said bath being greater than in cryolite.

17. In the electrolytic refining of aluminum, the step comprisingelectrolyzing an impure molten aluminum-bearing metal or alloy ofrelatively high density as anode, with a'su rimposed molten bathcontaining barium fl il brid bet-wen 30 and 38 per cent, sodium fluoridbetween 25 and 30 per cent, and aluminum fluorid between 30 and 38 percent.

18. Inthe electrolytic refining of aluminum, the step comprisingelectrolyzing an impure -molten aluminum-bearing metal or alloy ofrelatively high density as anode, with a superimposed molten bath ofapproximately the following ualitative and quantitative composition;barium fluorid between 30 and 38 per cent, sodium fluorid between 25 and30 per cent, aluminum fluorid between 30 'and 38 per cent, and aluminain amount less than suflieient to saturate the bath at the workingtemperature.

19. In the refining of aluminum, the step which consists in subjectingto electrolysis a combination of aluminum with a heavier metalin themolten state as anode, and substantially'pure aluminum as cathode, saidanode and cathode being gravitat-ively separated by an electrolytecontaining cryolitc and a heavy fluorid of a metal not more readilydeposited than aluminum in amount sufficient, to raise the specificgravity of the electrolyte without unduly decreasing its solvent powerfor alumina or its electrical conductivity, or increasing its freezingpoint.

20. In a process of electrolytically refining aluminum alloys with afused electrolyte, compounding the electrolyte to give it a higherdensity than aluminum and a selective action in dissolving aluminum fromthe alloy.

21. In a process of electrolytically refining aluminum alloys with afused electrolyte, compounding the electrolyte to give it a higherdensity than aluminum and a selective action in dissolving aluminum fromthe alloy, and deposit-ing from said bath upon a suitable cathode,aluminum of a high degree of purity.

22. In a process of electrolytically refining aluminum alloys with afusedelectrolyte, compounding the electrolyte so as to limit theproduction of sodium at the cathode.

\ 23. In a proccss of electrolytically refining aluminum alloys with afused eleetrol te, wherein current-carrying cans are emplbyed in contactwith the mol en cathode, compounding the electrolyte to minimize theproduction at the cathode of substances which attack thecurrent-carrying means, whereby the useful life of the latter issubstantially prolonged.

24. In a process of electrolytically refining aluminum alloys with afused electrolyte, wherein a molten anode, electrolyte, and cathode arearranged in gravitatively separated layers, with current-carrying meansin contact with the cathode, compounding the electrolyte to limit theproduction at the cathode of substances which attack saidourrent-carrying means, and to maintain the gravitative separation ofsaid layers.

25. In a process of electrolytically refining aluminum alloys with afused electrolyte, wherein "a molten anode, electrolyte, and cathode arearranged in gravitatively separated layers, compounding the electrolyteto limit the production of sodium at the cathode and maintain thegravitative sepathe cathode and give the electrolyte a selective actionin dissolving aluminum from the anode, and a density adapted to maintainthe gravitative separation of said layers.

27. In a process of electrolytically refining aluminum alloys with afused electrolyte, limiting the introduction of oxygen into theelectrolyte by compounding the electrolyte so as to limit the productionof metallic sodium at the cathode.

28. In a process of electrolytically refining aluminum alloys with afused electrolyte, compounding the electrolyte to give the same, whenfused, a greater density than molten aluminum at the same temperatureand to remain fluid at permissible working temperatures withoutexcessive volatilization.

29. An electrolyte composition for the purpose described containingaluminum and sodium fluoride, and .between 20 and160 per cent,approximately, of fluorid of an alkali earth metal having anatomic'weight greater than 80.

30. An electrolyte composition for the purpose described, substantiallyfreefrom chlorid, containing aluminum and sodium fluorids, and between20 and 60 per cent,

approximately, of fluorid of an alkali earth metal having an atomicweight greater than 80. p

31. An electrolyte composition for the purpose described composedessentially of fluorids and substantially free from other compoundsexcept oxid, and containing aluminum and sodium fluorids, and betweenand 60 per cent, approximately, of fluori-d of an alkali earth metalhaving an atomic weight greater than 80.

32. An electrolyte composition for the purpose described containingaluminum and sodiumifluorids and between 20 and 60 per cent,approximately, of barium fluorid.

33. An electrolyte composition for the purpose described containingaluminum and sodium fiuorids and between 20 and 60 per cent,approximately, of fiuorid of an alkali earth metal having an atomicweight greater than 80, and less than about 2 per cent of fluorid of analkali earth metal having an atomic weight below 80.

34. An electrol' composition for the purpose describe containin thefollowmg compounds in approximately the proportions named: alumlnumfiuorid 30 to 38 per cent, sodium fluorid to per cent, and bariumfluorid 30 to 38 per cent. 35. An electrolyte composition for thepurpose described containing aluminum and sodium fiuorids and between 20and 60 per cent, approximately, of fluorid of an'alkali earth metal vhavlng an atomic weight greater than 80,with less than about 7 per vcent of alumina. v A

v36. An electrolyte composition for the purpose described containin thefollowing compounds in approximate y the propor tions named: alumlnumfluorid 30 to 38 per cent, sodium fluorid 25 to 30 per cent, and

barium fluorid 30 to 38 per cent; with less than about 7 per cent ofalumina.

In testimony whereof we hereto afiix our signatures.

. WILLIAM HOOPES.

FRANCIS C. FRARY. JUNIUS D. EDWARDS.

